I do quite of bit of tile work and never use anything but a tile backer. The cement board is good. 1/4 inch over 5/8 plywood should be fine. There are other types of backer board that may be easier to cut cleanly for the sink opening. One of the problems with using any type of wood based substrate directly under the tile is that it will suck the moisture out of the thinset adhesive quickly. It may actually skim over before you get to set some of the tile if you are not quick enough and they will not get a very good bond. Also make sure you use thinset to set the backer board in and then screw it to the underlayment.

If you really need a clean cut in the durock, use a dry diamond blade in the cutting tool of your choice (4" angle grinder, circ saw, etc) it will also come in handy if you need to make any 'odd' cuts in the tile - ie anything besides a 'score-and-snap' cut.

I use 3/4" plywood followed by 1/2" duroc (cement board). If you want to avoid problems later, you want the substrate to be rock solid with no flex. As Mike notes, be sure to use thin set and screws to adhere the ducoc to the plywood. There are special screws for duroc. Ask for them when you buy the duroc. There is also a special hand cutter for the duroc (looks something like a linoleum cutter). You need to score both sides and then snap the duroc like you do with drywall. You can dress the edge with an old oil stone. Cutting it this way, you'll avoid all the dust which a skill saw would produce. You can cut out the hole for a sink in the plywood but the duroc is a bit too fragile for this. Just adhere pieces of scrap duroc to the front and back of the sink cut-out. Good luck.